Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated sheet metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. The most common metal workpieces used today in the automotive industry are those made of steel and an aluminum alloy. The desire to incorporate aluminum alloys into a vehicle has made it enviable to spot weld an aluminum alloy workpiece to another aluminum alloy workpiece or, alternatively, to a steel workpiece.
The resistance spot welding process is performed by an automated robotic or pedestal welding gun that includes two arms. Each of these arms holds a welding electrode typically comprised of a suitable copper alloy. The welding gun arms can be positioned on opposite sides of a workpiece stack-up and clamped to press the two electrodes against their respective metal workpieces at diametrically common spots. A momentary electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of electrical current through the metal workpieces and their faying interface (i.e., the contacting interface of the metal workpieces) generates heat at the faying interface. This heat forms a molten weld pool which, upon stoppage of the current flow, solidifies into a weld nugget. After the spot weld is formed, the welding arms release their clamping force, and the spot welding process is repeated at another weld site.
The electric current that is passed between the opposed electrodes and through the metal workpieces is received from a DC power supply carried by the welding gun. The DC power supply may, for example, be a medium-frequency integrated transformer and rectifier package configured to deliver high DC amperage in accordance with a specified weld schedule. This type of DC power supply, and other similar types as well, furnishes the opposed electrodes with fixed opposite polarities when electrically connected to the welding gun; that is, after the DC power supply has been installed, one electrode is always the positive electrode and the other is always the negative electrode.
The polarity assigned to the welding electrodes is not inconsequential. It has been found, for instance, that a polarity bias exists when spot welding (1) an aluminum alloy workpiece to another aluminum alloy workpiece, and (2) an aluminum alloy workpiece to a steel workpiece. A less pronounced polarity bias also exists when spot welding a steel workpiece to another steel workpiece and in certain practices of projection welding. The ability to control which electrode has the positive/negative polarity while the welding gun and the DC power supply remain electrically connected—including the ability to switch electrode polarities at any time—would permit more operationally effective spot welding practices to be developed in at least these instances, and possibly others. Such electrode polarity control cannot be achieved with conventional DC power supplies. In fact, when a conventional DC power supply is employed, the only way to change the polarity of the electrodes is to physically disconnect the power supply from the welding gun, and then re-connect the power supply in reverse polarity orientation, which is a time-consuming and laborious process.